A cellulose acylate film has been conventionally used for photographic supports or various optical materials because of its toughness and flame retardancy. Particularly, in recent years, it has been often used as an optical transparent film for a liquid crystal display apparatus. The cellulose acylate film is excellent as an optical material for apparatuses using polarized light, such as liquid crystal display apparatuses because of its high optical transparency and high optical isotropy. Accordingly, it has been so far used as a protective film of a polarizer or as a support of an optically-compensatory film capable of improving the display viewed from an oblique direction (viewing angle compensation).
In a polarizing plate which is one of members for a liquid crystal display apparatus, a protective film of a polarizer is formed by bonding on at least one side of the polarizer. A common polarizer can be obtained by dying a stretched polyvinyl alcohol (PVA) type film with iodine or a dichroic colorant. In many cases, as the protective film of the polarizer, a cellulose acylate film, especially a triacetyl cellulose film which can be directly bonded with respect to PVA is used. It is important that the protective film of the polarizer is excellent in optical isotropy. The optical characteristics of the protective film of the polarizer largely control the characteristics of the polarizing plate.
In recent liquid crystal display apparatuses, improvement of viewing angle characteristics has come to be more strongly demanded. Optical transparent films such as a protective film of a polarizer and a support of an optically-compensatory film are required to be optically more isotropic. In order to be optically isotropic, it is important that the retardation value denoted by the product of birefringence and thickness of the optical film is small. Particularly, for improving the display viewed from an oblique direction, not only the in-plane retardation (Re) but also the retardation (Rth) in a thickness-direction are required to be small. Specifically, it is required that, when the optical characteristics of an optical transparent film is evaluated, Re measured from the film front is small, and that the Re does not change even when measured by changing the angle.
There is proposed an optical transparent film showing small changes in angle of Re using a polycarbonate type film or a thermoplastic cycloolefin film in place of a cellulose acylate film (e.g., JP-A-2001-318233 and JP-A-2002-328233, as products, ZEONOR (manufactured by ZEON Corporation), and ARTON (manufactured by JSR), and the like). However, these optical transparent films have a problem in the bonding property with PVA when each film is used as a protective film of a polarizer because the film is hydrophobic. Further, there also remains a problem that the optical characteristics in the entire film plane are nonuniform.
As the solving means therefor, it is strongly demanded that a cellulose acylate film excellent in bonding suitability to PVA is more reduced in optical anisotropy, and improved. Specifically, there is a demand for an optically isotropic optical transparent film, a cellulose acylate film with an in-plane retardation Re set at roughly zero, and further showing small changes in angle of retardation, namely, with a Rth also set at zero.
In manufacturing a cellulose acylate film, generally, a compound called a plasticizer is added in order to improve the film forming performance. As the types of the plasticizer, there are disclosed phosphoric acid triesters such as phosphoric acid triphenyl and biphenyl diphenyl phosphate, and phthalic acid esters (e.g., see, Plastic Zairyou Kouza Vol. 17, Nikkan Kogyo Shimbun Ltd. “Fibrin resin” p. 121, (1970)). Some of these plasticizers are known to have an effect of reducing the optical anisotropy of the cellulose acylate film. For example, specific fatty acid esters are disclosed (e.g., see, JP-A-2001-247717). However, the conventionally known effect of reducing the optical anisotropy of the cellulose acylate film by the use of these compounds cannot be said to be sufficient.
Whereas, in a recent liquid crystal display apparatus, display tint is also required to be improved. For this reason, an optical transparent film such as a protective film of a polarizer or a support of an optically-compensatory film is required not only to be reduced in Re or Rth in a visible region at a wavelength of 400 to 700 nm, but also to be controlled in the changes in Re or Rth according to the wavelength, namely, the wavelength dispersion.
In JP-A-2006-030937, there are disclosed a cellulose acylate film using various low molecular weight Rth-reducing compounds, and a manufacturing method thereof. However, unfavorably, in solution film forming steps, in a step of volatilizing a solvent, the solvent volatilizes, and some deposited solvent in the step falls/drops on the film to cause failures, excess cost is taken in order to separate the volatilized product dissolved in the collected solvent, or in the evaluation of the durability of the film, the additive bleeds out, resulting in a failure. Further, when a saponification treatment is carried out for bonding with a polarizer, unfavorably, the additives flow out, and the optical characteristics of the film change unless the saponification step is stable.
In JP-A-2006-064803, there is disclosed a stretched cellulose ester film containing polyester polyol. In this case, there is a problem that the optical anisotropy is large, and thus, it is not possible to implement a cellulose ester film having a small optical anisotropy.
In WO 07/000,910, there is disclosed a stretched cellulose ester film containing polyester polyol with the main chain terminal sealed by monobasic acid or monoalcohol. In this case, particularly, in the case of a cellulose acylate film, there is a concern on the bleed-out from the film or the durability.